Process for preparing a sintered agglomerate



PROCESS FOR PREPARING ASINTERED AGGLOMERATE John C. Douglas, Katonah, N.Y., assignor to Union Carbide Corporation, a corporation of New York No Drawing. Application September 3, 1958 Serial No. 758,686

4 Claims. (Cl. 75-5) This invention relates to a chromium-containing alloy addition agent which is suitable for the production of steel in an electric furnace, to a process for preparing the chromium-containing addition agent and to a process for preparing chromium-containing steels in an electric furnace.

It is common practice to produce chromium-containing steels in an electric furnace by adding chromium in the form of retro-chromium alloys to an electric furnace charge of ferrous scrap. Another method comprises introdueing chromium ore together with ferro-silicon or a ferro-chromium silicon into a melted and decarburized bath of steel. Chromium-containing ferro alloys are commonly produced by the reduction of chromium ores. While these several processes have been used with varying degrees of success, certain disadvantages are attendant with each.

The ele'ctro-reduction of chromium-bearing material to a state where the chromium may be utilized in the forma-' tion of the chromium alloys necessitates the expenditure of large quantities of electrical energy. Additionally, an exceedingly large volume of slag is produced in the furnace which tends to further decrease the capacity ofthe electric furnace.

To employ chromium alloys as the addition agents has been found to be extremely'expensive in view of the fact that large quantities of electrical-energy areinvolved in the production of the chromium alloys in a reasonably pure form.

It is an object of this invention to provide an improved chromium-containing agglomerate which is suitable for use in the electric furnace production of alloys ,such as steel.

It is another object of this invention to provide a process for the preparation of the above-described agglomerate.

Still another object of the invention is to provide a process for the production of chromium-containing ferroalloys in the electric furnace.

Other objects will be apparent from the subsequent disclosure and appended claims.

The objects are achieved by providing a substantially uniform, sintered agglomerate consisting essentially of oxides and carbides of iron and chromium and free carbon, and having a composition such that from about 15 to 40 weight percent of the total composition is combined chromium, of which about 15 to 80 weight percent is acid soluble, the remainder of the, combined chromium being acid insoluble, from about 12 to 60 weight percent of the total composition is combined iron, of which at least 85 weight percent is acid soluble, there'mainder of. the combined iron being acidinsoluble, ,sufiicient .free carbon-so that the total carbon content is at least about 5- weight percent in excess over that necessary to provide a 1 to 1 mole ratio of carbon to oxygen in the agglomerate, and the remainder being the carbon and oxygen combined with the iron and chromium in the agglomerate and incidental impurities.

As used herein, the term acid soluble and acid-insoluble refer to the degree" of solubility of the material in'aqueous sulfuric acid. The major acid insoluble form of the iron and the chromium of the agglomerate' is believed to be ferrous chromite, FeOzCr' Og. The acid soluble forms are primarily carbides.

Included in the agglomerate are minor amounts of elemental iron in globule form which tend to form during the come of manufacturing the agglomerate.

The product of the present invention may be prepared by forming a comminuted mixture of chrome ore, iro'n ore and carbon in proportions sufiicient to produce the desired product, heating the mixture in an inert atmosphere at a temperature in the range of from 1000 C. to about 1300" C. and partially reducing the iron oxides and chromium oxides to the corresponding carbides in the solid state. If desired, the comminuted mixture may be mixed with a suitable noninterfering binder and pelletized prior to the heating step.

The original reaction mixture can be determined by simple stoichiometrical calculations.

The initial reduction, to produce the novel agglomerate of the present invention, may be carried out under substantially a carbon monoxide atmosphere. However, other inert gases such as nitrogen and argon may be present to dilute the carbon monoxide which is formed during the partial reduction reaction. Similarly, a vacuum may be applied to the reaction mixture thereby increasing the rate of reaction by drawing oif the formed carbon monoxide. 1 i

In an example of the present invention 96.0 pounds of a Transvaal chrome ore having the composition shown in Table I, 69.0 pounds of Brazilian hematite ore having the composition shown in Table II and 35.0 pounds of dried petroleum coke having the compositionshown in Table III, were ground to 100 mesh, mixed ,with 2.5 pounds of bentonite, 1.0 pound of channel carbon black and sufficient water for mixing.

TABLE. I

Transvaal chrome are i Percent- Cr O V 43.41 FeO v I 24.06 A1 0 14.94 MgO+CaO V 11.09 SiO 4.43

TABLE II Brazilian hematite are I v Fe O 97.10 SiO 0.97 A1 0 1.60 CaO+MgO 0.20 S 0.011 P 0.029

TABLEIII Y Dried petroleum coke Fixed carbon 87.80 Volatile matter 11.94 Ash p i 0.26

The resulting mixture was pelletized into pellets of approximately 1% inches and charged into a-vacuum fur- 1i Argon was added to the furnace to displace the carbon:

monoxide evolved. At the end of the reduction period the furnace was put on a twenty-two hour cooling cycle at the end of which the temperature was 200 C. The furnace was opened, the pellets removed, Weighed and analyzed. Results of the analyses indicated a combined chromium content of 17.85 percent, of which 15.46 percent was acid soluble, a combined iron content of 41.66 percent of which 92.61 percent was acid soluble, and a free carbon content of 10.75 percent.

To illustrate the utility of this material, 30 pounds of the pellets having the above analysis were added to a magnesia-lined, induction furnace containing 25 pounds of molten iron heated to a temperature of approximately 600 C. After all of the pellets had been fed to the bath the slag was, removed and a sample of the bath was analyzed. The sample consisted of 13.29 percent chromium, 2.50 percent carbon, 0.02 percent silicon, the remainder being iron and incidental impurities.

In another example of the invention, 71.0 pounds of Transvaal chrome ore having the composition shown in- Table I and 84.0 pounds of Brazilian hematite ore having the composition shown in Table II, 39.0 pounds of Pocahontas coal having the composition shown in Table IV were ground to 100 mesh and mixed with 2.5 pounds of bentonite, 1.0 part of channel carbon black and sufficient water for mixing.

TABLE IV Pocahontas coal Percent Fixed carbon 75.37 Volatile matter 16.94 Ash -2 7.69

The mixture was pressed into 1% inch almond-shaped pellets having a bulk density of 90 pounds per cubic foot and a moisture content of 1.40 percent. The pellets were heated in a mufiie type furnace in an atmosphere of carbon monoxide at a temperature of approximately 1300 C. for about six hours. The product had the following analysis: a combined chromium content of 18.42 percent, of which 53.51 percent was acid soluble, a combined iron content of 52.08 percent, of which 96.55 per: cent was acid soluble,'and a free carbon content of 3.90 percent.

To illustrate the direct electric furnace reduction of the partially reduced pellets, 2500 pounds of product having the above analysis and 30 pounds of quartz, were fed to an electric furnace operated with a submerged are at about 350 kilowatt load at 70 to 80 volts and 4000 to 5000 amperes. The furnace-operated exceptionally well and the metal was readily tapped and was very fluid. Analysis of the alloy and slag produced in the furnace were as shown in Table V.

TABLE V Percent Slag Percent by Alloy by Weight Weight 34.60 SiOz 38.8 2. 59 Total Or 3. 9 6.18 MgO-i-CaO 6i. Alz 3 Remainder.

One of the advantages accruing from the use of the 'a'gglomerates of the present invention in the electric furnace production of chromium-bearing steel and ferroalloys is that low-grade, high gangue ores generally unsuitable for metallurgical processes may be employed. When it is further considered that the reduction of chrome oxides to chrome carbides and of iron oxides to iron carbides represents over 75 percent of the chemical energy required in the production of chromium, and over 40 percent of the total furnacing energy, it may be seen that the use of partially reduced product as an electric furnace feed material can increase the furnace capacity by about 65 percent over the direct reduction of the chrome oxides and iron oxides in the furnace.

Further, When the agglomerates are used directly in the production of steel, only a single slag-removal step is required. Of course, when theagglomerates are added in the production of ferro-alloys which will then be used in the production of steels, slag removal is necessary both during the ferro-alloy production and during the subsequent steel production.

What is claimed is:

1. A substantially uniform sintered agglomerate consisting essentially of oxides and carbides of iron and chromium and free carbon, having a composition such that from about 15 to 40 weight percent of the total com position is combined chromium of which from about 15 to weight percent is acid soluble and the remainder is acid insoluble, from about 12 to 60 weight percent of the total composition is combined iron of which at least weight percent is acid soluble and the remainder is acid insoluble, suflicient free carbon so that the total carbon content is at least about 5 weight percent in excess over that necessary to provide a l to l gram-atom ratio of carbon to oxygen in said agglomerate, the remainder being the carbon and oxygen combined with said iron and said chromium in said agglomerate, and incidental impurities.

2. A process for preparing a sintered agglomerate suitable for the electric furnace production of chromiumcontaining ferro alloys comprising preparing a comminuted mixture of chrome ore, iron ore, and carbon in suitable proportions to produce an agglomerate of iron and chromium oxides and carbides and free carbon having a composition such that from about 15 to 40 weight percent of the total composition is combined chromium of which from about 15 to 80 weight percent is acid soluble and the remainder is acid insoluble, from about 12 to 60 weight percent of the total composition is combined iron of which at least 85 weight percent is acid soluble and the remainder is acid insoluble, sufficient free carbon so that the total carbon content is at least about 5 weight percent in excess over that necessary to provide a 1 to 1 gram-atom ratio of carbon to oxygen in said agglomerate, the remainder being the carbon and oxygen combined with said iron and said chromium in said agglomerate, and incidental impurities; heating said comminuted mixture in an inert atmosphere at a temperature of from about 1000 C. up to about 1300 C. until the agglomerate contains at least about 2.25 percent up to about 33.0 weight percent of acid soluble combined chromium, and cooling the resulting agglomerate product.

3. A process in accordance with claim 2, wherein the comminuted mixture is mixed with a binding amount of a non-interfering binder and formed into agglomerates prior to the heating step.

4. A process for preparing ferro-alloys which comprises preparing a comminuted mixture of chrome ore, iron ore, and carbon in suitable proportions to produce an agglomerate of iron and chromium oxides and carhides and free carbon having a composition such that from about 15 to 40 weight percent of the total com: position is combined chromium of which from about 15 to 80 weight percent is acid soluble and the remainder is acid insoluble, from about 12 to 60 Weight percent of the total composition is combined iron of which at least 85 weight percent is acid soluble and the remainder is acid insoluble, suflicient tree carbon so that the total carbon content is at least about 5 weight percent in excess over that necessary to provide a 1 to 1 gram-atom ratio of carbon to oxygen in said agglomerate, the remainder being the carbon and oxygen combined with said iron and said chromium in said agglomerate, and incidental 5 impurities; heating said comminuted mixture in an inert atmosphere at a temperature of from about 1000 C. up to about 1300 C. until the agglomerate contains at least about 2.25 percent up to about 33.0 weight percent of acid soluble combined chromium; and adding the agglomerate to a bath of molten ferrous metal in an electric furnace and tapping and solidifying the molten chromium-containing ferrous metal to produce a chromiumcontaining ferro-alloy.

References Cited in the file of this patent UNITED STATES PATENTS 2,256,536 Udy Sept. 23, 1941 2,582,469 Udy Jan. 15, 1952 2,839,379 Erasmus June 17, 1958 

1. A SUBSTANTIALLY UNIFORM SINTERED AGGLOMERATE CONSISTING ESSENTIALLY OF OXIDES AND CARBIDES OF IRON AND CHROMIUM AND FREE CARBON, HAVING A COMPOSITION SUCH THAT FROM ABOUT 15 TO 40 WEIGHT PERCENT OF THE TOTAL COMPOSITION IS COMBINED CHROMIUM OF WHICH FROM ABOUT 15 TO 80 WEIGHT PERCENT IS ACID SOLUBLE AND THE REMAINDER IS ACID INSOLUBLE, FROM ABOUT 12 TO 60 WEIGHT PERCENT OF THE TOTAL COMPOSITION IS COMBINED IRON OF WHICH AT LEAST 85 WEIGHT PERCENT IS ACID SOLUBLE AND THE REMAINDER IS ACID INSOLUBLE, SUFFICIENT FREE CARBON SO THAT THE TOTAL CARBON CONTENT IS AT LEAST ABOUT 5 WEIGHT PERCENT IN EXCESS OVER THAT NECESSARY TO PROVIDE A 1 TO 1 GRAM-ATOM RATIO OF CARBON TO OXYGEN IN SAID AGGLOMERATE, THE REMAINDER BEING THE CARBON AND OXYGEN COMBINED WITH SAID IRON AND SAID CHROMIUM IN SAID AGGLOMERATE, AND INCIDENTAL IMPURITIES. 